1. Field of the Invention
The invention concerns an aircraft with engine pods located outboard on the wings and tiltable about a transverse axis.
2. Prior Art
In the case of V/STOL aircraft with propeller propulsion, two configuration possibilities have, essentially, become known, i.e. tilting wing and tilting pod aircraft.
In tilting wing aircraft, the wing, on which the propeller engines are located, is rotated in total into a vertical position when taking off and landing so that the downward directed propeller jet acts as an upward thrust so that it can lift the aircraft weight. The fuselage axis and the wing section axis then form an approximate right angle.
When changing from hovering flight to cruise flight (the so-called transition), the wings are gradually swivelled from the vertical position to the normal horizontal position; vice versa during landing. It is evident that during transition, the wings cause a strong downwind over the whole span due to the very large angles of incidence and this downwind can affect the elevators. In this case, the elevators are subjected to a downward airflow which would again lead to a perturbation of the moment balance about the y (pitch) axis. In order to avoid this undesirable effect, the tailplanes or the front tandem wings are swivelled along with the wings in the previously known tilting wing aircraft, i.e. in the types LTV XC-142a, Canadair CL 84, the VFW project VC 400 and the Bolkow project BO 140. Even if the swivel angle of the tailplane can then be less than that of the wing, such a swivelling device which, furthermore, has to operate exactly synchronously with the swivelling of the wings, involves a substantial extra amount of weight and complexity.
Another example of a tilting wing aircraft is described in DE-PS 1,241,270. In this case, the wings are tilted over their whole span width for vertical take-off or hover. For this flight condition, a special jet nozzle unit is provided in the aircraft tail so that pitch control can be carried out when there is no or only a little forward speed. Because, due to the ground effect, the ailerons located on the trailing edge of the wings are not fully effective for yaw control about the aircraft vertical axis, the patent specification quoted proposes that the whole of the outer ends of the wings should be designed as lateral control surfaces located essentially completely within the air jet generated by the propeller propulsion.
Another disadvantage of the tilting wing configuration is the fact that during transition, the swivelling mechanism must accept all the loads from the propulsion system (inertia/gyroscropic torques, torques from propeller thrust) and due to the wings. Because the wings can build up substantial aerodynamic forces during transition and because load factors have to be taken into account, the swivelling mechanism must have correspondingly large dimensions and this costs space and weight.
A tilting wing aircraft does, in fact, usually provide usable cruise performance and it also offers a low frontal area during vertical take-off and landing. In addition, it can also be used for horizontal take-off and landing if required. On the other hand, however, these advantages are outweighed by the disadvantages mentioned above with respect to the complicated swivelling system and controls for the tailplane, which usually result in poor hovering flight properties.
In the tilting pod configuration similar to the known Bell XV-3 and XV-15 types, it is not the wing but only the propeller or rotor pod which is swivelled into the vertical position about the fixed wing. Another example of swivellable or tiltable rotors is described in DE-PS 2,032,259. In that case, the engine rotors are arranged to be tiltable at the end of fixed wings and are driven by engines provided in the aircraft tail. The rotors are designed in the manner of helicopters and can, by swivelling or tilting, be brought from helicopter operation with vertical take-off and vertical landing into propeller operation for cruise flight and vice versa. The rotors must then have full cyclic control with flapping hinges, etc. Special means are provided in order to provide the rotors, as required, with more or less of the properties of a rigid or a non-rigid rotor.
Although this avoids the disadvantages mentioned above of the tilting wing configuration with respect to detrimental tailplane effects and the complicated swivelling mechanism, other disadvantages appear--a major proportion of the fixed wing is subjected to the propeller jet in hover, i.e. the jet area loading (aircraft weight/propeller (rotor) area) must be reduced to an amount tolerable for hovering flight. It has been found in practice that the jet area loading leads into the range of magnitude usual for helicopters and this is in turn associated with the necessity for large rotors and the problems which then arise - high weight, design limitations (large rotor diameter), aeroelastic loading, gust sensitivity in particular and poor cruise flight performance because the rotor is too large. The shape of the wing is then orientated towards the propeller diameter and is not optimum for cruise flight. In addition, there is a high bearing load at the ends of the wings during rotor swivelling and this has to be accepted by relatively small trunnions located there. In addition, a normal landing with unswivelled pods is not possible in an emergency and the large rotors demand a complex design of the rotor head, similar to that for helicopters with full cyclic pitch control.